Production method of iron carbide

ABSTRACT

Disclosed is a method for producing iron carbide, in which metallic carbide is difficult to be formed on the inside of heating tubes of a tube-shaped heater for heating reducing gas and carburizing gas to be supplied to a reactor. Carburizing gas which is supplied to reactor  1  is heated in tube-shaped heater  6  by combustion gas used for heating reducing gas and circulating gas, and then a mixture comprising carburizing gas, reducing gas and circulating gas is supplied to reactor  1 . Iron-containing raw materials are reduced and carburized in reactor  1.

DESCRIPTION

1. Technical Field

The present invention relates to a method for producing iron carbidesuitable for raw materials for iron making and steel making whichcomprises iron crabide (Fe₃C) as a main component, for example, rawmaterials for steel making which is used in an electric furnace and thelike.

2. Background Art

The production of steel normally comprises the steps of converting ironore into pig iron using a blast furnace, and then converting the pigiron into steel using an open hearth furnace or a convertor. Such atraditional method requires large amounts of energy and large-scaleequipment, and has a high cost. Therefore, for a small-scale steelmaking, a method comprising the steps of directly converting iron oreinto raw materials to be used in a steel-making furnace, and convertingthe raw materials into steel using an electric furnace and the like hasbeen used. With respect to this direct steel making process, a directreduction process has been used to convert iron ore into reduced iron.However, the reduced iron produced by the direct reduction process ishighly reactive and reacts with oxygen in the air to generate heat.Therefore, it is necessary to seal the reduced iron with an inert gas orby some other measures during transportation and storage of the reducediron. Accordingly, iron carbide (Fe₃C) containing a comparatively highiron (Fe) content, and which has a low reaction activity and can beeasily transported and stored, has recently been used as the rawmaterials for steel making in an electric furnace and the like.

Furthermore, raw materials for iron making or steel making containingiron carbide as a main component is not only easy to be transported andstored, but also has the advantage that carbon element combined withiron element can be used as a source of fuel in an iron making or steelmaking furnace, and can be used as a source to generate microbubbleswhich accelerates a reaction in the steel making furnace. Therefore, rawmaterials for iron making or steel making containing iron carbide as amain component have recently attracted special interest.

According to a conventional method for producing iron carbide, iron orefines are fed into a fluidized bed reactor or the like, and are causedto react with a gas mixture comprising a reducing gas (e. g., hydrogengas) and a carburizing gas (e. g., methane gas and the like) at apredetermined temperature. Thus, iron oxides (e.g., hematite (Fe₂O₃),magnetite (Fe₃O₄), wustite (FeO)) contained in iron ore are reduced andcarburized in a single process (which means a process performed bysimultaneously introducing a reducing gas and a carburizing gas to asingle reator). This reaction is performed by the following overallreaction formula (1).

3Fe₂O₃+5H₂+2CH_(4→2)Fe₃C+9H₂O  (1)

The prior art related to the field of the present invention has beendescribed, for example, in the publication No. 6-501983 of Japanesetranslation of International Application (PCT/US91/05198).

In order to easily understand the present invention, an example of anapparatus for producing iron carbide according to the prior art will bedescribed below. For example, an apparatus shown in FIG. 3 has beenknown. With reference to FIG. 3, the reference number 1 denotes areactor. Iron-containing raw materials are fed into reactor 1 throughfeeding port 2 and iron carbide is discharged from exhaust port 3. Thereference number 4, 5 and 6 indicate a dehumidifier, a compressor, atube-shaped heater respectively. Reactor 1, dehumidifier 4, compressor5, and heater 6 form a circulating loop 7. The reference number 8 is aline for supplying natural gas containing methane as a main component.Line 8 diverges into line 9 and line 10, and line 9 is connected tocirculating loop 7 in the rear of compressor 5. Line 10 is connected tocirculating loop 7 via steam reformer 11, shift-converter 12, anddecarbonator 13. An example of method for producing iron carbide usingthe above apparatus will be described below.

When iron-containing raw materials for iron making are fed into reactor1 through feeding port 2, iron-containing raw materials are reduced andcarburized in reactor 1 to be converted into iron carbide in accordancewith the above reaction formula (1). In this reaction, since hydrogen isconsumed to perform a reducing reaction and methane is consumed toperform a carburizing reaction, it is necessary to supply reactor 1 withreducing gas component and carburizing gas component. So, natural gascontaining methane as the main component is supplied as carburizing gascomponent to circulating loop 7 through line 9.

Natural gas flowing through line 10 is steam-reformed according to thefollowing reaction formula (2) at steam reformer 11.

CH₄+H₂O→3H₂+CO₂  (2)

Carbon monoxide contained in steam-reformed gas is converted intohydrogen and carbon dioxide at shift converter 12 in accordance with thefollowing reaction formula (3)

CO+H₂OH₂+CO₂  (3)

Carbon dioxide obtained by the reaction at shift converter 12 iseliminated from the gas at decarbonator 13. Thus, hydrogen is suppliedfrom line 10 to circulating loop 7.

As described above, hydrogen and methane supplied to circulating loop 7are heated at tube-shaped heater 6 to a temperature of 650˜700° C. withcirculation gas circulating through loop 7. But, if gas containinghydrogen and methane is heated to such a high temperature, hydrocarbon(C_(n)H_(m)) comprising methane as the main component isthermal-decomposed and active carbon is generated according to thefollowing reaction formula (4). As shown in the following reactionformula (5), this active carbon is reacted with metallic component (M),such as Nickel and the like, which is material for heating tube to be aconstituent component of heater. As a result, metallic carbide (M_(x)C)is formed.

C_(n)H_(m)→nC+(m/2)H₂  (4)

C+_(x)M→M_(x)C  (5)

FIG. 5 shows a cementation speed of 20Cr—12Ni steel as an example ofcementation of metal under the conditions of temperature of 750° C.,pressure of 4˜6 atm., and reaction gas comprising a mixture of CH₄, CO,CO₂, H₂ and H₂O. In FIG. 5, line A of the graph denotes the case inwhich CH₄ accounts for 60 volume percent in the mixture of the abovegases, and line B of the graph denotes the case in which CH₄ accountsfor 65 volume percent in the mixture of the above gases. As shown inFIG. 5, a cementation speed is in the range from about 2.1 to 6.0mg/cm².60 hr.

However, if the formation of metallic carbide becomes a supersaturatedcondition, carbon is separated from metallic carbide as shown in thefollowing reaction formula (6). At this time, the metallic component onthe inside of heating tube 14 shown in FIG. 4 exfoliates and it ispossible that phenomena such as the decrease in thickness of heatingtube or pitting will occur.

M_(x)C→xM+C  (6)

In consideration of the above-mentioned problems of the prior art, it isan object of the present invention to provide a method for producingiron carbide in which metallic carbide is difficult to be formed on theinside of heating tube of a tube-shaped heater which heats reducing gasand carburizing gas for supplying to a reactor.

DISCLOSURE OF INVENTION

In order to accomplish the above-mentioned object, the present inventionis characterized by that carburizing gas is heated separately fromreducing gas or circulating gas and heated to lower temperature thanreducing gas or circulating gas. As a result, in accordance with thepresent invention, it is possible to control the formation of activecarbon in a tube-shaped heater and decrease exfoliation of metalliccomponent of heating tube due to separation of carbon.

A first aspect of the present invention is directed to a method forproducing iron carbide comprising the steps of heating hydrocarbon gas,hydrogen gas, which have been supplied from the outside, and acirculating gas at a tube-shaped heater, and then supplying the saidgases to a reactor in order to convert iron ore into iron carbide,wherein the only hydrocarbon gas is heated to lower temperature thanthermal decomposition temperature of the hydrocarbon gas at a separateheating tube from heating tubes for the other reaction gases, and thenmixed with the heated hydrogen gas and circulating gas in order tosupply to the reactor without further heating.

A second aspect of the present invention is directed to a method forproducing iron carbide comprising the step of supplying hydrocarbon gas,hydrogen gas, which have been supplied from the outside, and acirculating gas to a reactor in order to convert iron ore into ironcarbide, wherein the hydrocarbon gas is mixed with the other reactiongases, which have been heated at a tube-shaped heater, in order tosupply to the reactor.

A third aspect of the present invention is directed to a method forproducing iron carbide comprising the steps of heating hydrocarbon gas,hydrogen gas, which have been supplied from the outside, and acirculating gas at a tube-shaped heater, and then supplying the saidgases to a reactor in order to convert iron ore into iron carbide,wherein a mixture of the hydrocarbon gas and part of circulating gas isheated to lower temperature than thermal decomposition temperature ofthe hydrocarbon gas at a separate heating tube from heating tubes forthe other reaction gases and then mixed with the heated hydrogen gas andcirculating gas in order to supply to the reactor without furtherheating.

It is preferable that the temperature of the hydrocarbon gas, which havebeen supplied from the outside, at inside wall of the heating tube islower than thermal decomposition temperature of the hydrocarbon gas, andmore preferably the said temperature is in the temperature range from350 to 650° C.

In accordance with the present invention, iron-containing raw materialsfor iron making are feeded to a reactor. The iron-containing rawmaterials are reduced and carburized by reducing gas (hydrogen gas) andcarburizing gas (hydrocarbon gas), which have been heated up to apredetermined temperature in a heater and supplied to the reactor, to beconverted into iron carbide (Fe₃C). And then the iron carbide isdischarged from an exhaust port of the reactor. The gas after reactionin the reactor is circulated through a circulating loop. Since thecertain quantity of reducing gas and carburizing gas is consumed in thisreaction, a predetermined quantity of reducing gas component andcarburizing gas component is supplied to this reaction system. However,if carburizing gas is heated to a high temperature, active carbon isgenerated owing to thermal decomposition of hydrocarbon contained incarburizing gas. The above active carbon is reacted with metalliccomponent which is material for heating tube to be a constituentcomponent of heater to form metallic carbide. If the formation ofmetallic carbide becomes a supersaturated condition, carbon is separatedfrom metallic carbide. At this time, the metallic component of heatingtube may exfoliate.

But, in accordance with the present invention, since carburizing gas (or a mixture of the hydrocarbon gas and part of circulating gas) isheated separately from circulating gas or reducing gas, it is possibleto form no metallic carbide on the heating tube without thermaldecomposition of carburizing gas during heating the carburizing gas.Especially, if the heating temperature of carburizing gas is in thetemperature range from 350 to 650° C., the formation of metallic carbidebecomes less than that made under the other temperatures. Accordingly,it is possible to extend a life of heating tube.

In accordance with the present invention, since metallic carbide iscontrolled to be formed on the inside of tube-shaped heaters which heatreducing gas and carburizing gas to be supplied to a reactor, themetallic component of heating tube does not exfoliate and it is possibleto extend a life of heating tube.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a preferred example of anapparatus suitable for performing the method for producing iron carbideaccording to the present invention;

FIG. 2 is a partly enlarged view of tube-shaped heater shown in FIG. 1;

FIG. 3 is a schematic diagram showing an example of an apparatus forproducing iron carbide according to the prior art;

FIG. 4 is a partly enlarged view of tube-shaped heater shown in FIG. 3;

FIG. 5 is an example of cementation speed of metal.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a preferred embodiment of the present invention will bedescribed. FIG. 1 is a schematic diagram showing a preferred example ofapparatus suitable for performing the method for producing iron carbideaccording to the present invention. FIG. 1 shares with the samereference number as shown in FIG. 3. FIG. 1 is different from FIG. 3 inthat line 9 a diverged from line 8 for supplying natural gas isconnected to the rear of tube-shaped heater 6. That is to say, as shownin FIG. 2, natural gas flowing through line 9 a is heated at a separateheating tube in heater 6 from heating tube 14 through which circulatinggas and reducing gas flow. As described above, since line 9 a is heatedby combustion gas (G) used for heating heating tube 14, a temperature ofline 9 a is kept in the range of a temperature of 350˜650° C. As aresult, natural gas flowing through line 9 a is difficult to causethermal decomposition, thereby controlling the generation of activecarbon. Accordingly, generation of metallic carbide owing to cementationof the insides of line 9 a and heating tube 14 is controlled, and themetallic component of line 9 a and heating tube 14 does not exfoliate.

Furthermore, in FIG. 1, it is possible to mix part of circulating gaswith natural gas flowing through line 9 a and heat line 9 a bycombustion gas (G) used for heating heating tube 14, as described above.

Also, in FIG. 1, it is possible to make line 9 a bypass heater 6 withoutgoing through tube-shaped heater 6 (no heating of gas contained in line9 a at heater 6) and connect line 9 a to the rear of heater 6.

INDUSTRIAL APPLICABILITY

Since the present invention has the above-mentioned constitution, theapparatus in accordance with the present invention is suitable to anapparartus for producing iron carbide, in which metallic carbide isdifficult to be formed on the inside of heating tube of tube-shapedheater for heating reducing gas and carburizing gas to be supplied to areactor.

What is claimed is:
 1. A method for avoiding metallic carbide formationon the inside of a heating tube of a tube-shaped heater applied to ironcarbide producing process comprising the steps of heating hydrocarbongas, hydrogen gas, which has been supplied from the outside, andcirculating gas in the tube-shaped heater, and then transporting thegasses to a reactor in order to convert iron ore into iron carbide,wherein the hydrocarbon gas is heated to a lower temperature than thethermal decomposition temperature of the hydrocarbon gas in a separateheating tube from the heating tubes for the hydrogen and circulatinggases, mixed with the heated hydrogen gas and circulating gas andtransported to the reactor without further heating.
 2. A method foravoiding metallic carbide formation on the inside of a heating tube of atube-shaped heater applied to iron carbide producing process comprisingthe steps of heating hydrocarbon gas, hydrogen gas, which has beensupplied from the outside, and circulating gas in the tube-shapedheater, and then transporting the gases to a reactor to convert iron oreinto iron carbide, wherein a mixture of the hydrocarbon gas and part ofthe circulating gas is heated to a lower temperature than the thermaldecomposition temperature of the hydrocarbon gas in a separate heatingtube from the heating tubes for the hydrogen and circulating gases,mixed with the heated hydrogen gas and circulating gas and transportedto the reactor without further heating.